The Black Ring Sight Gives you a better overall sight picture then the White one. I put the Black on my PS90 and installed a red T3b to it. There is not a lot of sun here in the North East and too much snow in the winter so the Black crosshairs show up better.
I do install the T3b for free if you send me the Ring Sight.

I had the blue on white. Loved it.
But if you have both...that's a tough call. I know that installing them on the black is a bit more work...putting them on the white ring sites takes all of..what? 30 seconds?